This invention relates to an exhaust gas catalytic converter for a Diesel engine. More particularly, this invention relates to a catalytic converter of the type just described comprising means for enriching Diesel engine exhaust with hydrocarbons that support conversion of nitrogen oxides in the catalyst.
This application claims the priority of German Application No. 19617563.1-13, filed in Germany on May 2, 1996, the disclosure(s) of which is (are) expressly incorporated by reference herein.
It is known that conversion, i.e. the reduction of nitrogen oxides contained in the exhaust, is more difficult in Diesel engines by comparison with four-cycle engines by the fact that these engines are not operated around the stoichiometric point of the fuel/air ratio but always with a significant oxygen surplus. Therefore, means are known for enriching Diesel engine exhaust with hydrocarbons that support NOx conversion in the Diesel catalytic converter. For example, there has been proposed an independent injection device for injecting hydrocarbons into the exhaust line. Another proposal takes the form of a secondary injection process in which additional fuel is injected into the combustion chamber after the combustion process proper and then enters the exhaust line unburned. This HC enrichment makes it possible to improve the activity of a Diesel catalytic converter, designed for example as a so-called DeNOx catalytic converter, as far as NOx conversion is concerned within a limited temperature range, whose position on the temperature scale depends on the selected catalyst material and the chosen catalyst loading density. In four-cycle engines on the other hand NOx conversion remains approximately constant above a certain minimum temperature.
Published European application 0 622 107 A2 describes a two-stage exhaust catalytic converter for a Diesel engine in which a front catalytic converter stage (looking in the direction of flow of the exhaust) is designed for low-temperature conversion of hydrocarbons and soluble organic components and is charged with a platinum catalyst for this purpose, while a downstream catalytic converter stage is designed for high-temperature conversion of hydrocarbons and soluble organic components and is charged with a palladium catalyst for this purpose.
To improve exhaust catalytic converter function during cold starting operating phases, especially in four-cycle engines, it is known to provide an HC adsorber material in at least one of the catalytic converter stages. In this way, the hydrocarbons contained in the exhaust, which cannot be completely converted in the cold starting phase because catalyst activity is still limited at this time, are given intermediate storage so that later, after the cold starting phase is over, they can be desorbed and converted. Such systems are disclosed for example in German OS's 43 26 120 A1, 44 19 776 A1, and 44 36 754 A1 as well as in U.S. Pat. No. 5,296,198.
DE 40 32 085 A1 teaches a catalytic converter for reducing nitrogen oxides in exhaust, said converter incorporating at least two catalyst beds connected in series looking in the flow direction of the exhaust, the catalyst beds consisting of different catalyst materials for nitrogen oxide reduction. The different catalyst materials produce their strongest catalytic effects in different, adjacent sections of the relevant exhaust temperature range.